The present invention relates to methods of calibration of magnification of measuring microscopes.
Measuring microscopes are used in different areas of science and industry for measurements of sizes of various objects which have sizes from inches to angstroms. There is no microscope which is capable to measure objects in such a broad range of sizes. For measurements in centimeter, millimeter and micrometer subranges, optical measuring microscopes are the most convenient microscopes. The subrange of micrometer and nanometer sizes is covered by scanning electron measuring microscopes. In the area of nanometer, sub-nanometer and atom (angstrom) sizes, probe microscopes are utilized, such as scanning tunnel microscopes and atomic force microscopes. There are objects with intermediate sizes, which can be measured by microscopes of one type or another type. Typical examples are the features of modern integrating circuits which have submicron and nanometer sizes. These objects can be also measured with scanning electron microscopes or probe microscopes, depending on the nature and properties of the features to be measured. In these cases it is in principl important that the results of measurement of the same object be identical, regardless of the type of microscope which is used for the measurements. The required coincidence of the results of measurements can be provided only when scales of magnification of all used microscopes are coordinated with one another, and ratios of such scales are established with a high accuracy.
This problem is very difficult to solve since there is no universal calibrating standard which would be suitable for calibration of optical measuring microscopes, scanning electron measuring microscopes and probe microscopes. For example the probe microscopes, due to their exceptionally high resolution, are calibrated with the use of a reference sample which is created by nature, in particular a pattern of atoms on the surface of monocrystals. This reference sample is however completely unsuitable for calibration of optical measuring microscopes and scanning electron measuring microscopes, since the resolution of the microscopes of this types is lower than required for visualization of the atom pattern, so that this pattern is not shown on the images of the microscopes of these two types and can not serve as a common standard.
Known attempts to use as a universal reference sample so-called “hand-made” objects have not been successful. U.S. Pat. No. 5,825,670 formally declares a possibility of use of a conventional diffraction grating as a reference sample for the probe microscope. However, it is stated in the patent that the topography of the grating must be preliminarily (in other words before calibration of the microscopes) determined with high accuracy. The reference does not however mention any method of characterization (i.e. measurement) of the topography of the diffraction grating. For such measurements, in turn, it is necessary to have a microscope which is preliminarily precisely calibrated. Therefore the possibility of use of diffraction grating as a reference sample for the probe microscope which is mentioned in this reference is just a declaration without a specific support. The obstacle in this approach is that the diffraction grating to be used can never be completely uniform: the pitch of the diffraction grating does not remain constant and it somewhat changes from one point of observation to another. This causes non-reproducibility of the calibration and significantly reduces its accuracy.
Coordination of scales of magnification of microscopes of any type can be provided only if all of the microscopes are calibrated in a single, absolute scale of sizes.